In this context, we extended the direct synthesis of TMOS
using silica derived from various natural resources, such as rice
experiments were systematically carried out in 200 mL and 1 L
batch reactor scale. For 200 mL-scale synthesis, Wakogel was
2
6
hull, pampas grass, rice straw and bamboo (Sasa). These silica-
rich waste coproducts of agricultural industries have the
potential to provide a silica-based commodity. The raw materials
were calcined at various temperatures (500–1000 °C) prior to use
to eliminate basic components that could inhibit the reaction.
used as SiO
1-4). Obviously, the TMOS yield increased steadily with
increasing CO pressure, and reaction at 0.8 MPa yielded TMOS
52% (Entry 3). The presence of CO is, again, crucial for TMOS
2 2
source under various CO initial pressures (Entries
2
2
synthesis, since in the reaction under Ar, TMOS yield drastically
dropped to 2% (Entry 4). Finally, synthesis of TMOS was carried
The SiO
straw and bamboo (Sasa) determined by EDXRF were > 90%,
5%, 84%, and 73%, respectively. The surface areas of these pre-
treated materials were determined by N adsorption–desorption
2
contents of rice hull ash (RHA), pampas grass, rice
out in 1 L batch reactor using Wakogel and RHA-500 as SiO
2
8
source (Entries 5 and 6). Increasing the synthesis scale to 1 L
showed no depletion of TMOS yield, as the TMOS yield can be
achieved to 63% and 69% using Wakogel and RHA-500,
respectively (Entries 5,6).
2
isotherms (using the BET method), as shown in Table 1. Notably,
the surface area of RHAs decreased with increasing calcination
temperature and after the reaction (Entries 2–7). Based on x-ray
diffraction (XRD) results, the amorphous SiO
2
from RHA was
4. Conclusion
transformed to the α-cristobalite crystalline phase (JCPDS 39-
A simple and practical method to synthesize TMOS via a direct
transformation of silica with methanol was achieved using a base
catalyst and organic dehydrating agents. The kind of the
dehydrating agent was crucial factor in the production of TMOS,
with 2,2-dimethoxypropane identified as the best dehydrant for
optimal TMOS yields. In fact, we found that the TMOS yield
was directly proportional to DMC generation. We proposed that
2
4
1
425) after being annealed to > 900 °C (Figure 5). These
changes due to calcined treatment are further reflected in the
observed reactivity. The RHA samples with higher surface areas
afforded higher TMOS yields, and the higher crystallinity
resulting from higher calcination temperatures produced lower
TMOS yields (Entries 2–7). Low surface area rice straw-500 and
bamboo-500, however, generated high TMOS yields (37% and
the TMOS production was promoted by CO
situ generation of DMC, which enhanced the depolymerization
rate of SiO . Therefore, this reaction system not only
demonstrates the potential application of SiO feedstock but also
2
arising from the in-
4
4%, respectively) due to the increased surface area during the
reaction, which was confirmed by the surface area of recovered
SiO after the reaction (Entries 9 and 10). Low surface area and
2
2
2
high crystallinity were presumably responsible for poor
dissolution rates which lowered the depolymerization velocity.
This methodology provides an alternative pathway for TROS
synthesis from the carbothermal reduction (1900 °C) to Simet
followed by ROH treatment.
introduces a new synthetic approach for the application of CO
as a benign catalyst.
2
Acknowledgement
This research was financially supported by the Development of
Innovative Catalytic Processes for Organosilicon Functional
Materials Project of NEDO, Japan.
Supporting Information
2 2
Photographs of natural SiO resources and effect of CO pressure,
6
*
= Cristoballite
*
5
4
3
2
1
0
o
Rice hull 1000 C
*
*
*
amount of acetal, and base catalysts on the TMOS yield. This
material is available on https://doi.org/10.1246/bcsj.****
o
Rice hull 900 C
References
o
Rice hull 800 C
1.
L. L. Hench, J. K. West, Chem. Rev. 1990, 90, 33.
2
3
4
.
.
.
M. Alagar, V. Krishnasamy, Chem. Eng. Commun. 1989,
8
0, 1.
o
Rice hull 500 C
J. M. Roberts, J. L. Placke, D. V. Eldred, D. E. Katsoulis,
Ind. Eng. Chem. Res. 2017, 56, 11652-11655.
J. M. Roberts, D. V. Eldred, D. E. Katsoulis, Ind. Eng.
Chem. Res. 2016, 55, 1813-1818.
5
15
25
35
θ/Degree
45
55
65
2
Figure 5. XRD pattern of RHAs with various calcination
temperatures.
5
6
.
.
E. G. Rochow, U.S. Pat. Appl. US 2,473,260, 1949
R. M. Laine, J. C. Furgal, P. Doan, D. Pan, V. Popova, X.
Zhang, Angew. Chem. Int. Ed. 2016, 55, 1065.
A. Rosenheim, B. Raibmann, G. Schendel, Z. Anorg. Allg.
Chem. 1931, 196, 160.
3
.5 Results of scaling-up synthesis of TMOS
7
.
Table 2. Scaling-up of TMOS Synthesis
b
c
SiO
2
Reactor
/mL
SiO
2
CO
2
TMOS
/ %
33
Entry
8.
R. M. Laine, K. Y. Blohowiak, T. R. Robinson, M. L.
Hoppe, P. Nardi, J. Kampf, J. Uhm, Nature 1991, 353, 642.
D. L. Bailey, F. M. O'connor, U.S. Pat. Appl. US 2,881,198,
source
Wakogel
Wakogel
Wakogel
Wakogel
Wakogel
RHA-500
/mmol
30
/MPa
0.1
1
2
3
200
9
1
1
1
.
200
30
0.4
49
1
959.
0. G. B. Goodwin, M. E. Kenney, Inorg. Chem. 1990, 29,
216.
1. Y. Ono, M. Akiyama, E. Suzuki, Chem. Mater. 1993, 5,
42.
2. L. N. Lewis, F. J. Schattenmann, T. M. Jordan, J. C.
Carnahan, W. P. Flanagan, R. J. Wroczynski, J. P. Lemmon,
J. M. Anostario, M. A. Othon, Inorg. Chem. 2002, 41, 2608.
3. J. C. Choi, L. N. He, H. Yasuda, T. Sakakura, Green Chem.
200
30
0.8
52
a
4
200
30
0.8
2
1
5
6
1000
1000
150
0.8
63
150
0.8
69
4
a
c
. b Intial pressure at 25 °C.
Ar gas was used instead of CO
Yield based on SiO
2
2
.
Although our work has demonstrated the promising
1
1
prospect for direct synthesis of TMOS using silica from natural
resources, further investigation is necessary to evaluate their
feasibility in large-scale application. In this study, scale-up
2
002, 4, 230.
4. J. C. Choi, K. Kohno, Y. Ohshima, H. Yasuda, T. Sakakura,
Catal. Commun. 2008, 9, 1630.